The life-cycle of Apatemon gracilis (Rud., 1819) Szidat, 1928 was maintained in the laboratory using domestic chicks, Lymnaea peregra and rainbow trout. This achievement enabled the infection, when required, of rainbow trout with cercariae released by patent snails. Comparisons were made of size, morphology and egg content of adult worms obtained from a variety of avian hosts. The growth and development of metacercariae in the pericardial cavity of experimentally infected rainbow trout was monitored over a period of twelve weeks. The pathology associated with the host response to this infection was also studied. Accumulation of host tissue around the parasite cysts in the pericardial cavity occurs and, in old infections, there are frequently collagenous adhesions which attach the heart to the inner wall of the pericardial cavity. In vitro studies of cardiac performance revealed that, although cardiac output of infected hearts increases with preload (venous) pressure, the cardiac output of infected hearts constituted only 20-40% of that of healthy, uninfected hearts. Correspondingly, the power output of infected hearts was only, at best, 41% of that of the control hearts. The implications of these findings for the swimming performance and maximum oxygen consumption of infected fish are discussed. The electrical properties of the heart, as assessed by the recording of electrocardiograms from intact fish, were found to be affected by the infection. Short-term experiments carried out to examine the differences between growth rates of healthy and infected fish at early and late stages of infection, showed that the growth of infected fish does not appear to be adversely affected. Apatemon gracilis metacercarial cysts were found in rainbow trout sampled from five Scottish fish farms. This distribution coincides with that of sawbills (red-breasted mergansers and goosanders) in Scotland; these birds probably act as the definitive host in the life-cycle of Apatemon gracilis.